Signals may be modulated using phase-shift keying (PSK) and amplitude-shift keying (ASK) modulation to communicate data. In PSK modulation, changes in the phase of a signal are used to represent data, and in ASK modulation, changes in the amplitude of a signal are used to represent data.
A transmitter may include PSK and ASK modulators. The PSK modulator translates a bit sequence into signal phase changes that represent the bit sequence, and the ASK modulator translates another bit sequence into signal amplitudes that represent the bit sequence. A receiver may include PSK and ASK demodulators. The PSK demodulator translates the phase changes and the ASK demodulator translates the amplitudes to retrieve the bit sequences.
In ASK modulation, an extinction ratio refers to the ratio of the optical power of a “1” pulse to the optical power of a “0” pulse. In general, a higher extinction ratio may improve ASK communication. In ASK-PSK modulation, however, ASK communication may be optimal at an extinction ratio of 10 decibels, but PSK communication may not be optimal. Known systems typically select an extinction ratio that is a compromise between the optimal extinction ratio for ASK communication and the optimal extinction ratio for PSK communication. For example, an extinction ratio of 7 decibels may be selected. The compromise extinction ratio, however, is typically not optimal for either ASK or PSK communication.